This is mainly to avoid some warning when building the app, such as:
```
WARNING in .../angular/aio/src/environments/environment.archive.ts is
part of the TypeScript compilation but it's unused.
Add only entry points to the 'files' or 'include' properties in your
tsconfig.
```
(Not turning on `fullTemplateTypeCheck` due to lots of errors.)
PR Close#32923
The angular.io project uses Angular and CLI v9, which by default turns
on Ivy mode. However, since ec4381dd4, we explicitly opt out of Ivy.
This commit removes the `enabledIvy: false` configuration, thus allowing
the default behavior of having Ivy on.
NOTE:
This commit only changes the angular.io projects. The docs examples need
to be updated separately (first to Angular and CLI v9 and then to Ivy).
PR Close#32923
